Coupling vibrational transitions to resonant optical modes creates vibrational polaritons shifted from the uncoupled molecular resonances and provides a convenient way to modify the energetics of molecular vibrations. This approach is a viable method to explore controlling chemical reactivity. In this work, we report pump&ndash;probe infrared spectroscopy of the cavity-coupled C&ndash;O stretching band of W(CO)6 and the direct measurement of the lifetime of a vibration-cavity polariton. The upper polariton relaxes 10 times more quickly than the uncoupled vibrational mode. Tuning the polariton energy changes the polariton transient spectra and relaxation times. We also observe quantum beats, so-called vacuum Rabi oscillations, between the upper and lower vibration-cavity polaritons. In addition to establishing that coupling to an optical cavity modifies the energy-transfer dynamics of the coupled molecules, this work points out the possibility of systematic and predictive modification of the excited-state kinetics of vibration-cavity polariton systems.

f6: Vacuum Rabi oscillations in cavity-coupled W(CO)6.(a) Representative transient absorption traces highlighting the strong oscillatory signals evident when the pulses are nearly coincident in time. (b) Measured period of early-time oscillations as a function of the effective splitting extracted from the dispersion curve for each concentration. (c) Frequency calculated from the oscillation period for cavity-coupled W(CO)6 of varying concentration as a function of the measured splitting. The dashed line represents the case where the oscillation frequency and effective splitting are equal. Error bars represent s.d. from measurements of the period across each kinetic scan.

Mentions:
In systems that are strongly coupled, including cavity-coupled electronic excitations in semiconductors and plasmon polaritons coupled to organic dyes, early-time oscillations, whose frequency scales directly with effective splitting, have been observed and attributed to vacuum Rabi oscillations, that is, coherent exchange of energy between the coupled states154555. Figure 6a shows examples of the oscillatory signals we observe. The oscillations start at negative time delays when the probe pulse arrives at the sample before the excitation pulse, suggesting that the probe pulse generates a coherence that the excitation pulse interacts with, as observed in perturbed free-induction decays45565758. However, the periods of the oscillations we observe are insensitive to probe frequency or pump pulse intensity, which is in stark contrast to the highly probe-frequency dependent perturbed free-induction decays or uncoupled Rabi oscillations (‘flopping')59. Importantly, the frequency of the oscillations observed in the vibration-cavity polariton system scales linearly with, and is nearly equal to, the measured splitting (Fig. 6b,c), which we control by changing the W(CO)6 solution concentration. This equivalency is an important criterion for identifying vacuum Rabi oscillations and has been used to attribute similar oscillatory behaviour in cavity-coupled excitonic systems to vacuum Rabi oscillations2445. This notable observation is the first evidence for vacuum Rabi oscillations in a coupled vibration-cavity polariton system, to the best of our knowledge. We note that interference between two fields at the sample can also create such oscillations15, but measurements where the probe pulse is either parallel or perpendicular to the excitation pulse give identical oscillations, making us confident that the oscillations are indeed Rabi oscillations. Higher-order nonlinear spectroscopies can differentiate between mechanisms of coherent energy transfer and present an exciting avenue for future experiments60.

f6: Vacuum Rabi oscillations in cavity-coupled W(CO)6.(a) Representative transient absorption traces highlighting the strong oscillatory signals evident when the pulses are nearly coincident in time. (b) Measured period of early-time oscillations as a function of the effective splitting extracted from the dispersion curve for each concentration. (c) Frequency calculated from the oscillation period for cavity-coupled W(CO)6 of varying concentration as a function of the measured splitting. The dashed line represents the case where the oscillation frequency and effective splitting are equal. Error bars represent s.d. from measurements of the period across each kinetic scan.

Mentions:
In systems that are strongly coupled, including cavity-coupled electronic excitations in semiconductors and plasmon polaritons coupled to organic dyes, early-time oscillations, whose frequency scales directly with effective splitting, have been observed and attributed to vacuum Rabi oscillations, that is, coherent exchange of energy between the coupled states154555. Figure 6a shows examples of the oscillatory signals we observe. The oscillations start at negative time delays when the probe pulse arrives at the sample before the excitation pulse, suggesting that the probe pulse generates a coherence that the excitation pulse interacts with, as observed in perturbed free-induction decays45565758. However, the periods of the oscillations we observe are insensitive to probe frequency or pump pulse intensity, which is in stark contrast to the highly probe-frequency dependent perturbed free-induction decays or uncoupled Rabi oscillations (‘flopping')59. Importantly, the frequency of the oscillations observed in the vibration-cavity polariton system scales linearly with, and is nearly equal to, the measured splitting (Fig. 6b,c), which we control by changing the W(CO)6 solution concentration. This equivalency is an important criterion for identifying vacuum Rabi oscillations and has been used to attribute similar oscillatory behaviour in cavity-coupled excitonic systems to vacuum Rabi oscillations2445. This notable observation is the first evidence for vacuum Rabi oscillations in a coupled vibration-cavity polariton system, to the best of our knowledge. We note that interference between two fields at the sample can also create such oscillations15, but measurements where the probe pulse is either parallel or perpendicular to the excitation pulse give identical oscillations, making us confident that the oscillations are indeed Rabi oscillations. Higher-order nonlinear spectroscopies can differentiate between mechanisms of coherent energy transfer and present an exciting avenue for future experiments60.

Coupling vibrational transitions to resonant optical modes creates vibrational polaritons shifted from the uncoupled molecular resonances and provides a convenient way to modify the energetics of molecular vibrations. This approach is a viable method to explore controlling chemical reactivity. In this work, we report pump&ndash;probe infrared spectroscopy of the cavity-coupled C&ndash;O stretching band of W(CO)6 and the direct measurement of the lifetime of a vibration-cavity polariton. The upper polariton relaxes 10 times more quickly than the uncoupled vibrational mode. Tuning the polariton energy changes the polariton transient spectra and relaxation times. We also observe quantum beats, so-called vacuum Rabi oscillations, between the upper and lower vibration-cavity polaritons. In addition to establishing that coupling to an optical cavity modifies the energy-transfer dynamics of the coupled molecules, this work points out the possibility of systematic and predictive modification of the excited-state kinetics of vibration-cavity polariton systems.